A group of astronomers from all over the world now have the best view we've seen of two galaxies colliding, thanks to many telescopes, both on Earth and in space, and gravitational lensing.

Perhaps what is most interesting about this discovery is that it shows something that happened when the Universe was only about half its current age.

Astronomers found this galaxy collision due to an international effort that included telescopes that are both on Earth and in space, including the Hubble Space Telescope, the Atacama Large Millimeter/submillimeter Array (ALMA), the Keck Observatory and the Karl Jansky Very Large Array (VLA). However, an effect known as gravitational lensing also lent a hand.

Gravitational lensing happens when galaxies line up just so that any light hitting a galaxy bends around it due to strong gravitational forces. This creates a natural magnifying glass in space that allows astronomers to see what objects are behind that galaxy, as well as more remote areas, including those from when the Universe was younger.

"While astronomers are often limited by the power of their telescopes, in some cases our ability to see detail is hugely boosted by natural lenses, created by the Universe," says Hugo Messias of the Universidad de Concepcion in Chile and the Centro de Astronomia e Astrofisica da Universidade de Lisboa iN Lisbon. "Einstein predicted in his theory of general relativity that, given enough mass, light does not travel in a straight line but will be bent in a similar way to light refracted by a normal lens."

In this instance of gravitational lensing, astronomers spotted an object they call H-ATLAS J142935.3-002836. Although it appeared faint at first, it is still one of the most brightly magnified areas of space that we've found so far, even though it's from a time when the Universe was only about half the age it is now.

Further research combined multiple telescopes, which uncovered details of this object, which astronomers now believe are two galaxies colliding with each other. The ALMA telescope, in particular, which can trace carbon monoxide, even unveiled that one of these galaxies is still rotating, indicating that it was once disc-shaped, much like our Milky Way galaxy. It also showed that this collision is creating hundreds of new stars each year.

"ALMA enabled us to solve this conundrum because it gives us information about the velocity of the gas in the galaxies, which makes it possible to disentangle the various components, revealing the classic signature of a galaxy merger," says ESO's Director of Science Rob Ivison. "This beautiful study catches a galaxy merger red-handed as it triggers an extreme starburst."

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